The present invention relates to a magnetic recording medium and a magnetic storage apparatus and, more particularly, to a magnetic recording medium having a recording density not less than 50 giga bits (Gb)/in2 and a magnetic storage apparatus in which the magnetic recording medium is incorporated.
Since 1998, the areal recording density of a magnetic storage apparatus is raised by an increase rate of about 100%/year. However, as the areal recording density is raised, a problem of so-called thermal fluctuation comes to be caused in which magnetically recorded data disappear due to surrounding heat, so that the achieving of an areal recording density more than 50 Gb/in2 is considered to be difficult insofar as a conventional longitudinal recording method is concerned.
On the other hand, a perpendicular recording method has, unlike the longitudinal recording method, the characteristic that, the more the linear recording density is raised, the more a demagnetizing field occurring among adjacent bits decreases, so that a recorded magnetization can be maintained to be stable. Further, in the perpendicular recording method, a high head magnetic field can be obtained by providing below a perpendicular recording layer a soft magnetic underlayer having a high permeability, whereby it becomes possible to enhance the coercivity of the perpendicular recording layer. Because of these reasons, the perpendicular recording method is considered to be one of effective means for overcoming the thermal fluctuation limit occurring in the longitudinal recording method.
In the perpendicular recording method, the combination of a single-pole-type head and double-layer perpendicular recording media each comprising a soft magnetic underlayer and a perpendicular recording layer is effective in realizing the high desity recording. However, since the double-layer perpendicular recording medium has the soft magnetic underlayer having a high saturation magnetic flux density (Bs), there occur such problems as a stray field occurring from the domain walls of the soft magnetic underlayer is observed as spike noises and as the recorded magnetization disappears due to the domain wall motion. To overcome these problems, it is proposed to provide a hard magnetic pinning layer between the soft magnetic layer and a substrate so that the magnetization of the soft magnetic layer is directed in one direction, as proposed, for example, in JP-A-7-129946 and JP-A-11-191217. Further, as disclosed in JP-A-6-103553, another method is proposed in which the domain wall motion of the soft magnetic underlayer is suppressed by an exchange coupling occurring in connection with an antiferromagnetic layer in which the direction of the magnetic spin is set in one direction.
The inventors of the invention have found that the prior art method in which the magnetization of the soft magnetic underlayer is directed in one direction by the hard magnetic pinning layer is apt to cause magnetic domains of the reverse direction at the inner and outer edges of a disk substrate with result that the spike noise occurring from these portions is observed. The inventors of the invention have further found that, although the other prior art method in which the domain wall motion of the soft magnetic underlayer is suppressed by the antiferromagnetic layer is effective in suppressing the disappearance of the recorded magnetization caused by the domain wall motion, it is impossible to suppress the spike noise caused by the domain walls.
The invention is made to solve the above problem occurring in the prior arts. More specifically, the object of the invention is to provide a perpendicular magnetic recording medium having a recording density not less than 50 Gb/in2 and a high S/N ratio regarding the medium while suppressing the spike noise by controlling the magnetic domains of the soft magnetic underlayer, and to thereby make it easy to realize a magnetic storage apparatus of a high recording density.
According to the first aspect of the invention, there is provided a perpendicular magnetic recording medium comprising a non-magnetic layer having a face-centered cubic (, that is, fcc) structure, an antiferromagnetic layer provided on the non-magnetic layer, a soft magnetic underlayer provided on the antiferromagnetic layer, and a perpendicular recording layer provided above the soft magnetic underlayer.
A domain-control layer may be provided beneath the soft magnetic underlayer, whereby the spike noise from the soft magnetic underlayer is suppressed. The domain-control layer has a structure in which the ferromagnetic layer of a bcc structure is provided on the antiferromagnetic layer. As the material of the ferromagnetic layer of the bcc structure, there is preferred a material bringing about a large exchange coupling between the antiferromagnetic layer and the ferromagnetic layer, and more specifically it is possible to use a bcc alloy containing as the main constituent elements thereof Co (cobalt) and Fe (iron). In the composition of the CoFe alloy, the content of Fe may be not less than 20 atomic % but not more than 70 atomic % (, the atomic % being simply expressed below as xe2x80x9c%xe2x80x9d). Particularly, in a case where the content of Fe is not less than 30% but not more than 50%, the large exchange coupling can be obtained. The thickness of the crystalline ferromagnetic layer may be not less than 1 nm but not more than 10 nm, by which thickness range the crystal grains do not become coarse in size so much, so that the influence of the magneto-crystalline anisotropy can be suppressed.
By providing beneath the antiferromagnetic layer a non-magnetic layer having a fcc structure, it is possible to obtain such preferred effects as the crystalline texture of the antiferromagnetic layer is enhanced and as the grain size thereof can be controlled. As a specific material for the non-magnetic layer having the fcc structure, there may be used a fcc metal of the platinum group such as Pd (palladium), Pt (platinum) and Ir (iridium) etc. or a fcc alloy containing as the main constituent element thereof the metal of the platinum group. The non-magnetic layer having the fcc structure may be provided directly on the substrate, however, it is preferred to provide an amorphous or microcrystalline pre-coating layer between the substrate and the non-magnetic layer, which acts to enhance the adhesion of the layers.
It is preferred that the antiferromagnetic layer provided directly on the non-magnetic layer having the fcc structure is made of a material revealing an anti-ferromagnetism at the stage of forming the layer. Specifically, a disordered alloy can be used which contains Mn (manganese) and Ir as the main constituent elements thereof. In general, the ordered alloy such as a PtMn alloy and a NiMn alloy etc. is in a disordered state at film formation and does not reveal any anti-ferromagnetism. Thus, in order to obtain an exchange coupling between the antiferromagnetic layer and the crystalline ferromagnetic layer, it becomes necessary to perform an ordering heat treatment for several hours while applying a magnetic field. This step is not preferred because this step makes the recording medium-producing process complex and increases the production cost.
The material of the soft magnetic underlayer is not particularly limited insofar as the material having Bs not less than 1 Tesla (T), a coercivity not more than 1 Oe when measured in the direction of the travelling of a magnetic head and a superiority in surface flatness is concerned. Specifically, there can be used an amorphous or microcrystalline alloy containing as the main constituent element thereof Fe or Co and at least one additive selected from the group consisting of Ta, Hf, Nb, Zr, Si, B and C etc. Incidentally, in the SI unit system, 1 Oe is equivalent to 79.6 A/m.
The perpendicular recording layer provided above the soft magnetic underlayer through an intermediate layer can suppress the medium noise. The intermediate layer may be made of a non-magnetic alloy having an amorphous structure or an hexagonal closed packed (hcp) structure or a fcc structure. The intermediate layer may be a single-layer film or may be a bi-layer film comprising, for example, an amorphous alloy layer and a hcp alloy layer. The perpendicular recording layer may be made of a hcp-Co alloy such as an CoCrPt alloy, a CoCrPtB alloy and etc. or may be a multi-layer film of Co/Pd or Co/Pt.
According to the second aspects of the invention, there is provided a magnetic storage apparatus comprising the above-explained perpendicular magnetic recording medium, a driving unit for actuating the recording medium in a recording direction, a magnetic head having a recording section and a reproduction section, and means for relatively moving the magnetic head with respect to the perpendicular magnetic recording medium, recording and reproducing means for inputting the signals of the magnetic head or for performing the reproduction of the output signal from the magnetic head, the recording section of the magnetic head comprising a single-pole-type head, the reproduction section of the magnetic head being provided with a high sensitive element in which a magneto-resistive effect or a tunneling magneto-resistive effect is utilized, whereby it becomes possible to realize the magnetic storage apparatus having a recording density not less than 50 Gb/in2 and a high reliablilty.
In the magnetic storage apparatus, the magnetic head may be provided with, as the reproduction section, a sensitive element using a magnetoresistive effect or a tunneling magnetoresistive effect.
According to the third aspect of the invention, there is provided a magnetic storage apparatus comprising a perpendicular magnetic recording medium, and a magnetic head provided with a reproduction section and a recording section, the perpendicular recording medium comprising an antiferromagnetic layer, a crystalline ferromagnetic layer provided directly on the antiferromagnetic layer, a soft magnetic underlayer provided directly on the crystalline ferromagnetic layer, and a perpendicular recording layer provided on the soft magnetic underlayer, the crystalline ferromagnetic layer having a bcc structure, the magnetic head comprising a single-pole-type head as the recording section.
In the magnetic storage apparatus according to the third aspect of the invention, the magnetic head may have, as the reproduction section, a sensitive element using a magnetoresistive effect or a tunneling magnetoresistive effect.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.